Short answer: nobody knows for sure. Normally I don't trust wikipedia, but in this case it looks like they've done a good job summarising this story.
>Discussion of Die Glocke originated in the works of Igor Witkowski. His 2000 Polish language book Prawda o Wunderwaffe (The Truth About The Wonder Weapon, reprinted in German as Die Wahrheit über die Wunderwaffe), refers to it as "The Nazi-Bell".
>Witkowski wrote that he first discovered the existence of Die Glocke by reading transcripts from an interrogation of former Nazi SS Officer Jakob Sporrenberg. According to Witkowski, he was shown the allegedly classified transcripts in August 1997 by an unnamed Polish intelligence contact who said he had access to Polish government documents regarding Nazi secret weapons.[3]
>Witkowski maintains that he was only allowed to transcribe the documents and was not allowed to make any copies. Although no evidence of the veracity of Witkowski’s statements has been produced, they reached a wider audience when they were retold by British author Nick Cook, who added his own views to Witkowski’s statements in The Hunt for Zero Point.[4]
>Author Jason Colavito wrote that Witkowski's claims were "recycled" from 1960s rumors of Nazi occult science first published in Morning of the Magicians, and describes Die Glocke as "a device few outside of fringe culture think actually existed. In short, it looks to be a hoax, or at least a wild exaggeration."[5]
Witkowski's account doesn't sound very plausible to me, but that doesn't mean it's necessarily false. However, since he's the only primary source of the story, and we have no way of verifying it for ourselves, it comes down to whether or not he can be trusted. Is he a serious researcher who got lucky or a charlatan who made this stuff up to sell more copies?